Control apparatus



Feb. 25, 1941. L w so 2,232,896

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CONTROL APPARATUS Original Filed MarGh 21, 1940 8 Sheets-Sheet 8 .4150 r1: gm"

Patented Feb. 25, 1941 UNITED STATES PATENT OFFICE CONTROL APPARATUS Original application March 21, 1940, Serial No.

Divided and this application September 30, 1940, Serial No. 359,133

14 Claims.

This is a division of my copending application Serial No. 325,150, filed March 21, 1940.

This invention relates to apparatus for use in controlling the relative speeds of moving parts of machinery, and particularly to such apparatus for use in maintaining a substantially synchronous relation in the rotative speeds of two or more rotating parts such as engine or motor drive shafts and the like. The present invention constitutes an improvement upon certain aspects of the control apparatus shown and claimed in my co-pending application Serial No. 291,844.

It is frequently desirable to maintain the drive shafts of two or more engines or the like in substantial synchronism, as where such engines or the like are operated to supply power to a single operative device. Prior control apparatus for so correlating the associated drives on machinery of this character has contemplated setting up opposing mechanical, electromotive, or magnetic forces respectively proportional in magnitude to the rotative speeds of the parts to be synchronized or otherwise correlated, and the unbalancing of such opposing forces has been effective through the medium of devices such as differential governors, synchronous motors, differentially wound relays, or the like, to bring about a control operation of the apparatus.

It is a salient object of the present invention to enable the operation of two or more engines or like driving means to be correlated without resorting to the use of detecting apparatus responsive to the interaction of opposing forces.

In the development of aircraft such as airplanes it has been found advantageous in many instances to employ two or more engines, each driving a propeller shaft on the plane or other aircraft. One of the difiiculties which has been encountered on aircraft of this kind is that undesirable low-frequency beat vibrations may be set up when the several propeller shafts do not rotate in substantial synchronism with each other, and these beat-frequency vibrations are frequently manifested in various parts of the aircraft structure in the form of concentrated hamme ing impulses which have a more or less disturbing effect upon the occupants. Moreover, such beat vibrations are likely to appear in locations where they have a detrimental effect, such as in the wings of a plane, and thus they may tend to weaken the aircraft structure.

It has been proposed heretofore to automatically maintain two or more propellers on multi-engined airc aft and the like in substantial synchronism by providing a plurality of generators,

each driven by a propeller shaft, and electrically connecting the generators in opposition to each other in a series circuit including an electrically operable means such as a motor which functions in dependence upon the resultant electromotive force in the series circuit to vary the speed of an engine that is to be maintained in timed relation with a master engine. An outstanding disadvantage of such an arrangement is that the generator are comparatively heavy and thus 10 considerable extra weight is added to the aircraft structure. Moreover, it is relatively diflicult to design generators of small size whose electrical characteristics will be substantially identical at all speeds within the working range.

In view of the foregoing, it is an object of my invention to enable the operation of the propellers in aircraft and the like to be accurately controlled and maintained the same in substantial synchronism under all conditions which may be encountered in practice, by the use of apparatus which is of extremely light, simple and inexpensive construction.

A further object is to enable engine speed to be controlled, for example by automatically varying the propeller pitch, through the use of novel means responsive to the relative speeds of the propellers and having a time factor for enabling operation of such pitch-adjusting or other regulating means only upon the establishment of at least a predetermined difference in rotative speeds.

A still further object is to dispense with generators and the like for comparing propeller or propeller shaft speeds (such terms being used synonymously hereinafter) and to employ in lieu thereof electrical contact-making .devices, such as commutator switches, which are embodied in a control circuit including timing means, such as slow-to-release relays, that are responsive to said commutator switches for controlling the propeller pitch-adjusting means or other engine speed governing means in dependence upon whether or not there is at least a predetermined discrepancy in propeller speeds, and in accordance with the sense of the relative rotations of the propellers and in this regard sense of relative rotation is to be understood to mean the direction of rotation of one propeller relative to another or master propeller, or, to state this in an equivalent manner, the rotative speed of the first-named propeller in comparison with the speed of the master propeller. Thus, assuming the direction of rotation of both propellers to be clockwise as viewed from the rear of the aircraft, if the first propeller is rotating more slowly than is the master propeller, then the sense of relative rotation of the first propeller is counterclockwise, because this propeller is rotating clockwise less rapidly than is the master propeller. On the other hand, if the first propeller is rotating more rapidly than is the master propeller, the sense of relative rotation of the first propeller is clockwise, because this propeller is rotating clockwise at a greater speed than is the master propeller. It is in this manner that the Just explained term is used hereinafter.

A still further object is to electrically interconnect at least two relatively stationary comm'utators; to provide each commutator with one or more brushes or other contact-making means rotatively movable relative thereto in response to the rotative speed oi a respective one of at least two members which are to be operated in substantial synchronism; to establish energizing circuits through the commutator brushes to electrically operable means such as relays; to condition a selected one of still other energizing circuits to a further electrically operable means for conditioning the same in accordance with the sense of the relative rotations of said members; and to render said relays eifective to complete the selected one of the last-named energizing circuits whenever there is at least a predetermined variation in the relative speeds of the brushes of said commutators, said further electrically operated means operating when thus energized in a selected manner to bring one of said rotating members into substantial synchronism with the other or another of such members.

A further object is to provide means of the above described character for detecting the sense of the relative rotation, if any, between two independently rotatable parts of machinery or the like, and to provide further means for ascertaining when the magnitude of such relative rotation exceeds predetermined limits, said means and said further means being effective to control the rotative speed of one of said rotatable machine parts and to substantially synchronize it with the other of such rotating parts when a predetermined difference in rotative speeds arises.

Other and further objects of the present invention will be apparent from the following description and claims and are illustrated in the accompanying drawings which, by way of illustration, show preferred embodiments and the principle thereof and what I now consider to be the best mode in which I have contemplated ap plying that principle. Other embodiments of the invention embodying the same or equivalent principle may be used and structural changes may be made as desired by those skilled in the art without departing from the present invention and the purview of the appended claims.

In the drawings,

Fig. 1 is a schematic plan view of the nose portion of an airplane showing an embodiment of my invention as used in maintaining a timed relation in the operation of two aircraft engines;

Fig. 2 is a diagrammatic view of a motor which is responsive to my novel control apparatus for regulating engine speed;

Fig. 3 is a diagrammatic view of an indicating device which may be associated with the control apparatus;

Fig. 4 is a diagrammatic view of a series of commuator switches and circuit interconnections therefor employed in one embodiment of my invention;

Fig. 5 is a diagrammatic view of a control relay arrangement which may be utilized in con- Junction with the commutator switches shown in Fig. 4;

Figs. 6, 7 and 8 are diagrammatic views of a modified form of the invention shown in Figs. 4 and 5;

- Figs. 9, 10 and 11 are views respectively similar to Figs. 6, 7 and 8 and showing another modified form of my invention;

Figs. 12, 13 and 14 are further views respectively similar to Figs. 6,-7 and 8 and showing still another modified form of my invention;

Figs. 15, 16 and 17 are diagrammatic views of a modification similar to that shown in Figs. 9 to 11;

Fig. 18 is a wiring diagram of an embodiment in which the brushes for determining direction of relative rotation are driven independently of the brushes for effecting control when the magnitude of such relative rotation exceeds predetermined limits;

Fig. 19 is a diagrammatic view of an arrangement that might be used under certain conditions for reducing the number of conductors required in the cables that are used in conjunction with my apparatus; and

Figs. 20 and 21 are diagrammatic views of two modifications each similar to that shown in Figs. 9 to 11.

In the form of my invention which is adapted for maintaining a timed relation in the speeds of two or more engines on aircraft and the like, one of the engines, such as El, driving a propeller as Pl, Fig. 1, is selected to be the master engine, and the other engine or engines, such as E2, each driving a propeller as P2, are maintained in substantial synchronism with this master engine by the control apparatus. Briefly, such control apparatus comprises at least one master commutator switch as SI having rotative parts driven by the master engine El; another or other commutator switch or switches as 82, each having rotative elements which are driven by a. corresponding engine as E2 that is to be operated in timed relation with the master engine El a control unit as CU that is governed jointly by a master switch as SI and by a secondary switch as S2 for each engine as E2 that is to be so timed with respect to the engine El; and a motor or like means M for each engine as E2 and which is controlled by a control unit as CU for regulating the speed of the engine as E2 to maintain the same in substantial synchronism with the master engine El. Such regulation of the engine speed by the motor M can be accomplished in a number of ways, for example by affecting the rate at which fuel is supplied to the engine E2, but at present I prefer the well-known arrangement in which variable pitch propellers are employed on an airplane or other aircraft and wherein the speed of each engine may be affected in a desired amount by adjusting the pitch of the propeller driven thereby.

Referring to the wiring diagram of the motor M in Fig. 2, said motor is preferably of the direct-current type operable on low voltage and is provided with two alternatively energizable field coils 20 and II for driving the armature 22 of the motor M in either a forward or a reverse direction. When circuit is completed from a source of voltage through a conductor 23 to the field coil 2|, in a manner to be explained, and

thence through the winding 24 of a relay 25, conductor 26, rheostat 21, and conductor 28 back to the source of voltage, field coil 20 is energized for driving the motor M, say, forwardly, and at the same time the relay 25 closes its normally open contact 25A to complete circuit from the same or another voltage source through conductor 29, a normally closed'cam contact 30, conductor 3|, relay contact 25A, conductor 32, winding of armature 22, conductor 33, a normally closed cam contact 34, and conductor 35 back to the voltage source. Normally the motor M is then driven forwardly until such time as circuit to the field coil 20 is broken, whereupon relay 25 is deenergized and opens its contact 25A to break the circuit to the armature 22 and thereby stop the operation of the motor M, this arrangement insuring that the motor will have no tendency to race when the field is demagnetized. As the motor turns, it actuates a means which, in the present instance, causes a variation in the pitch of the propeller P2, Fig. 1, and such means may comprise, for example, a worm rack operable through suitable devices to change the pitch of the propeller and effect a corresponding adjustment of the engine speed, or the motor M can expeditiously serve to actuate a fluid pressure system such as is shown, for example, in United States Letters Patent No. 2,188,313, issued January 30, 1940, for varying the pitch of the propeller P2. Preferably the mechanical reduction ratio afforded by the pitch-adjusting means is great enough so that, in connection with certain other features of the control apparatus described hereinafter, it serves to insure that there will be no tendency for the device to hunt before attaining a condition of substantial synchronism.

In a manner fully described hereinafter,the field coil 20 of the motor M can normally be energized to cause the pitch of the propeller P2 to be varied in a predetermined direction only when the rotation of the propeller P2 relative to the propeller PI is in a certain sense, say counterclockwise, assuming the propellers PI and P2 to be rotating in a clockwise direction, as viewed from the rear looking toward the nose of the aircraft shown in Fig. 1, such energizatlon of the field coil 20 being effected under the control of the control unit CU, Fig. 1. However, when the rotation of the propeller P2 relative to the propeller PI is in a clockwise sense, still assuming both of the propellers PI and P2 to be rotating clockwise as viewed from the rear of the aircraft, the control unit CU may establish a circuit from the aforesaid source of voltage to a conductor 36, the field coil 2|, winding 24 of relay 25, conductor 26, rheostat 21, and conductor 28 back to the voltage source, and this is eiiective to drive the motor M in its reverse direction in a manner similar to that described hereinabove for forward rotation of the motor M. Thus the motor M can actuate the pitchadjusting means of the propeller P2 either in a forward or in a reverse direction for bringing the engine E2 into substantial synchronism with the engine El.

If, for example, it is required that the engine E2 be slowed down relative to the engine El, the pitch of the propeller P2 should be increased to thereby increase the load on the engine E2, and this is accomplished in the present instance by energizing the field coil 2| to drive the motor M in its reverse direction. On the other hand, if the speed of engine E2 is to be increased, the load on this engine is decreased by energizing the field coil 20 of the motor M to thereby drive the motor forwardly and thus decrease the pitch of the propeller P2 and consequently the load on the engine E2.

' The cam contacts 30 and 34 serve as limit switches for breaking circuit to the armature 22 in the event the propeller P2 is adjusted to one or the other of its extreme angular positions. It will be understood that while I have described the motor M as being a direct-current motor equipped with separate field coils for forward and reverse rotation, it would be equally feasible to employ a single field coil with a reversing switch arrangement that would be responsive to the alternative potentialization of the conductors 23 or 36. It is likewise manifest that an alternating-current motor could be employed in lieu of a direct current motor, the reversible shadingcoil type of motor being satisfactory for this purpose.

It may be desirable to furnish some indication to the pilot of the aircraft when the rotative speeds of the propellers as PI and P2 differ in a substantial amount and to also indicate in what sense the propeller as P2 is rotating relative to the master propeller as Pl. To this end, lamps 31 and 31', Fig. 3, may be provided on the instrument panel for each propeller as P2. Preferably these lamps are operated under the control unit CU, Fig. 1, and this may be accomplished by connecting one terminal of thefilament 38 of the lamp 3'! to the conductor 23, Fig. 2, by a conductor 39, the other terminal of the filament 38 being connected by a conductor 40 to one terminal 4| of the filament 42 of lamp 31', and thence through a conductor 43, Figs. 3 and 2, to the conductor 28. The other terminal of the filament 42 is connected by a conductor 44 to the conductor 36, and thus whenever circuit is closed by the control unit CU to the field coil 20, the lamp 3! will be lighted, but if circuit is closed to the field coll 2|, the lamp 3'! will be lighted. While I have illustrated a visual signal means for the purpose of indicating relative rotations of the propellers, it is equally expedient to employ an audible signal, or both.

Referring now to the embodiment of the invention shown in Figs. 4 and 5, each of the switches as SI and S2, Fig. 1, comprises a commutator generally designated 45 stationarily mounted on the frame of the associated engine as El, this commutator being composed of a number of segments 46 which are insulated from each other and are arranged around the circumference of a circle. The switch SI of the master engine El is provided with a single brush 4'! that is rotatably mounted so as to make wiping contact with the segments 46 of the commutator 45 of the switch Sl. Brush 41 is adapted to be driven by the propeller shaft of the master engine El either by being mounted directly thereon or by being geared thereto. The switch S2 includes a commutator 45 stationarily mounted on the engine E2 and rotatable cooperating brushes 48, 49 and 50 which are maintained in fixed circumferentially spaced relation to each other and are driven by the crank shaft of the engine E2. The commutators 45 and 45' of the switches Sl and S2 respectively are interconnected segment for segment by conductors included in a cable generally designated Figs. 1 and 4, which extends between the switches SI and S2.

If the aircraft includes further engines in additionto those shown in Fig. 1, each such engine will have associated therewith a commutator switchasB3,Sl,andsoonto SI, and alloithe commutators of these switches will be interconnected segment for segment with the commutator 45 of the switch SI in the same manner as is the commutator 45' of the switch 82, which is to say by conductors in the cable ll. Thus a flexible electrical connection is provided between the master switch SI and the switches as S2 of the engines as E2 that are to be substantially synchronized with the master engine El.

The brush 41 of the switch Si is maintained in electrical contact (for instance, by means of a slip ring or the like) with a conductor 52, Figs. 4 and 5, which leads to a terminal is that is connected by a conductor 54 to one terminal 55 ot a source of voltage B. The other terminal ll of the voltage source B is respectively connected by conductors 51 and 55 to one side of each of the windings 59 and I of a pair of slow-to-release relays SRI and SR2. The other side of the winding 55 of the relay SRI is connected by a conductor ii to the brush of the switch S2, Fig. 4. Similarly, the other side of the winding I of the relay SR2 is connected by a conductor 52 to the brush 5. 01' the switch S2. Brush ll is electrically connected through a conductor 53 to the terminal of a contact 6| 0! the relay SR2, which contact is normally in engagement with another contact 55 when relay SR2 is deenergized. Contact 55 is connected by a conductor 55 to one side of the winding 51 of a quick-acting relay R, and the other side of this winding is connected through a conductor ll to the terminal 5 and thence by a conductor III to the terminal 55 of the voltage source B.

Ii the propellers PI and P2, Fig. 1, are rotating in exact timed relation, there is no relative rotation of the brushes oi the switches SI and S2. However, when the propeller P2 rotates relative to the propeller Pl, there is relative rotation of the brushes 48, 49 and 50 of the switch S2 with respect to the brush ll of the switch Si and in this event the brush 4! is successively brought into electrical contact with the brushes ll, and 50, the sequence in which such contacts are made being determined by the sense oi the relative rotation of the propellers PI and P2, as will now be explained. Assuming that normally the direction of rotation of the propellers is such that the brushes of the switches as SI and S2 rotate clockwise as viewed in Fig. 4, if the propeller P2 rotates more slowly than does the master propeller Pl, the brushes of the switch S2 will rotate counterclockwise relative to the brush 4! of the switch SI. Under these circumstances, the brush 4! may first be brought into electrical contact with the brush 8 whereupon circuit is momentarily completed from the terminal 55 of the voltage source B through conductor 5|, terminal 53, conductor 52, brush ll, conductors of cable 5|, brush 4!, conductor Bl, winding 59 of the slow-to-release relay SRI, and conductor 51 to the terminal 56 oi the source B of electric current. Relay SRI thereupon energizes and closes its contact 1|. Continued counterclockwise relative rotation of the brushes oi! switch S2 causes the brush ll to be next brought into electrical contact with brush 9 and circuit is thereupon extended from the terminal 55 of the source B of electric current in a manner similar to that just described through the brush 4! and conductor 53 to the normally closed contacts 54 and 65 of the relay SR2 to the conductor 66 and thence through the winding 1 cl relay R, conductor 5|, terminal 5 and conductor II to the terminal 55 of the source B of electric current. Relay R thereupon energizes and remains energized so long as brush 41 remains in electrical contact with the brush 4!. Relay SRI, being slow to release, maintains its contact H closed for a predetermined length of time alter the brush 51 moves out of electrical contact with the brush ll; hence it the rate of relative rotation of the propeller P2 is great enough. the brush 41 of switch SI may be brought into electrical contact with the rush of switch S2 prior to the time when relay SRl releases its contact 1|. Brushes ll and are spaced comparatively close together about the periphery of the commutator l5 and the brush ll is of such width that it is brought into electrical contact with the brush 5| prior to moving out of electrical contact with the brush 4!. Therefore, relay R remains energized while circuit is completed from brush 41 through the brush 5! and conductor 52 to the winding 5| of relay SR2 and thence through conductor 5| to the terminal 55 of the source of electric current. Relay SR2 in energizing closes circuit between its contact 55 and another contact I2 prior to separating the contact 54 from the contact 85, the contacts "I2, 55 and I thus constituting make-beiore-break contacts. Such closure of the contact 55 with the contact I2 completes a circuit from terminal 55 of the voltage source B through a conductor 13 and a contact ll of the relay R, which contact is closed while relay R is energized, and conductor I5 through the contacts 12 and to the conductor 55 to thereby provide a holding circuit for the relay R so long as relay SR2 remains energized. When relay R was energized upon establishment of a circuit through brushes 4! and 49, it separated a movable contact Ii thereof from a stationary fixed contact TI, with which it is in engagement when relay R is deenergised, and brought this contact 16 into engagement with another stationary contact II which is connected to the conductor 23, Figs. 5 and 2. Energization of the relay SR2 in the manner described closes contact I! of this re lay and assuming that the degree of relative rotation of the propeller P2, Fig. 1, is suiliciently great that the relay SR2 is energized prior to the time relay SRI has released its contact 1i, circuit is completed from the terminal 55 01 the voltage source B, through conductor 54, terminal 53, conductor Ill, relay contact ll, conductor ll, contact I! of relay SR2, conductor 82 and contacts l5 and ll of the relay R to the conductor 23 and thence circuit is extended in the manner hereinabove described through the field coil 2| of the motor M to the conductor 25 which leads to the terminal 69, Fig. 5, that is connected by the conductor II to the terminal 5 of the volt age source B.

It will be seen from the foregoing that whenever the propeller P2 is rotating relative to the propeller Pl in such a. manner that the brushes of the switch S2 rotate counterclockwise relative to the brush 4! of the master switch SI, as viewed in Fig. 4, the relays SRi and SR2 are alternately energized and if the degree of relative rotation is such that relay SR2 is energized within the release time of the relay SRI an energizing circuit is completed through the contacts II and I! of the relays SRi and SR2 and the contacts 16 and 18 of the relay R to the ileld coil 2| of the motor M for thereby driving the motor M in a iorward direction, which in the present instance suflices to increase the speed of the engine E2 (either by reducing the pitch of the propeller P2 to lighten the load on the engine E2, or by regulating the speed of the engine E2 in any other equivalent manner) for thereby decreasing the counterclockwise relative rotation of the propeller P2 with respect to the propeller Pl, as viewed from the rear of the aircraft. Thus, the control unit CU, Fig. 5, is responsive to at least a predetermined variation in the rotative speed of the propeller P2 relative to the propeller Pl, as detected by means of the commutator switches as SI and S2, Fig. 4, for correcting the relative rotative speed of the propeller P2.

The brushes 48, 49 and 50, Fig. 4, continue to rotate counterclockwise relative to the brush 41 so long as there is any such relative rotation of the propeller P2, and as brush 4'! moves out of electrical contact with brush 50, the energizing circuit to the winding of the relay SR2 is broken to thereby initiate deenergization of this relay. If brush 4'! then electrically contacts brush 48 prior to the time relay SR2 has released its contact 19, the relay SR! is energized to effect or maintain closure, as the case may be, of the contact H thereof, and the above described energizing circuit to the winding 20 of the motor M continues.

For a reason which will appear hereinafter, the release times of the relays SRI and SR2 and the brush and segment widths are so proportioned that it will not be possible, upon initiation of such relative rotations as may be expected to develop in practice, for a brush as to move out of electrical contact with the master brush 4! and back into electrical contact with the master brush 4'! (at least during the first relative turn of the brushes as 50 with respect to the brush 4'!) within the period during which the relay as SR2 remains in an operative condition after it has been initially energized by the first electrical contact between brushes as 41 and 50. Thus, the average relative velocity of the brushes as 50 with respect to the master brush 4! must have at least a predetermined value in order that the relays SR! and SR2 may be maintained in a continual state of concurrent operation, and depending upon whether or not the relative acceleration persists long enough to produce this average relative velocity, an energizing circuit to the motor M will be either continuously or intermittently established by the relays SR! and SR2. In no event, however, will a relay as SR2 be maintained in an operative condition throughout the entire initial relative turn of a brush as 50 with respect to the master brush 4! in a given direction.

After the first rotation of the brushes as 50 relative to the master brush 41 has been completed, if relative acceleration of the propellers PI and P2 continues, this may cause the relative rotation to become great enough for the relays SRI and SR2 and R (still assuming counterclockwise relative rotation of the brushes 48, 49 and 50) to remain in a continual state of energization, relay R being held locked up through its aforesaid holding circuit controlled by relay SR2, and the field coil as 20 and the armature 22 of the motor M are therefore maintained energized for continuously operating the motor M until the speed of the engine E2 has been varied sufficiently to bring the average relative velocity of the propellers PI and P2 below the aforesaid value as determined by the time factor embodied in the slow-to-release relays SRI and SR2. Thereafter, the control unit CU continues to actuate the motor M for varying the velocity of the propeller P2 relative to the master propeller Pl, but in this instance circuit to a motor field coil as 20 and armature 22 cannot be maintained throughout the entire time interval between two successive electrical contacts of a brush as 41 with a brush as 50, inas much as one or the other of the relays SRI or SR2 will be released in this interval of time. Hence, as the brushes rotate relative to each other, an energizing circuit is repeatedly established by the relays SRI and SR2 to a motor field coil as 20. The frequency with which the motor M is thus repeatedly actuated obviously decreases while a condition of substantial synchronization is being approached, since the brush 4'! does not repeatedly contact the brushes 48 and 50 at as great a rate at low relative speeds. Thus, the motor M is comparatively infrequently actuated at low relative speeds approaching zero, and this, in combination with the high reduction ratio and negligible momentum of the motor drive, renders the likelihood of the apparatus hunting while the propellers are being brought into timed relation extremely remote.

It will be appreciated that there might be an instance in which the relays SRI, SR2 and R are energized for correcting relative rotation of the propeller P2 such as causes counterclockwise relative rotation of the brushes as 50, Fig. 4, as just described, and wherein substantial synchronization is restored within the time interval while the relays SRI and SR2 are still concurrently operative. However, this would only be likely to occur where the relative velocity was very small to begin with, and under such circumstances the overlap in release times of the relays SR! and SR2 is relatively slight, so that in addition to the fact that the motor M is infrequently energized when the propeller P2 is approaching substantial synchronism with the propeller Pl, the duration of each such energization period of the motor M becomes correspondingly small. In any event the propeller P2 would not be brought far enough past a condition of substantial synchronism to cause hunting of the control apparatus.

In the event the propeller P2 rotates more rapidly than the propeller Pl, still assuming that both propellers are rotating in such a direction as to drive the brushes of the switches SI and S2 clockwise as viewed in Fig. 4, the brushes 48, 49 and 50 of the switch S2 associated with the propeller P2 then rotate clockwise relative to the brush 4'! of the master switch SI. In this case the brush 4! may be brought into electrical contact first, with, say, the brush 48, thereby energizing the slow-to-release relay SR I If the clockwise relative rotation of the propeller P2 continues, the brush 4! then breaks contact with the brush 48 and is brought into electrical contact with the brush 50 to thereby energize the slow-to-release relay SR2. Continued relative rotation of the brushes causes the brush 49 to be brought into electrical contact with the brush 41 while the latter is still in electrical contact with the brush 50. When relay SR2 energizes, it separates its contacts 64 and 65 so that when circuit is extended from the voltage source B through brush 4! and the brush 49 to thereby apply potential on the conductor 63, there is no further effect in so far as the relay R is concerned because the connection from the conductor II to the conductor 99 which leads to the winding of the relay R is broken when the aforesaid contacts 94 and II are separated. Hence under these circumstances the relay B does not energize and the contacts 19 and 11 of this relay remain in engagement while the contact 19, of this relay remains separated from the contact 14. Assuming that the degree of relative rotation is such that the slow-to-release relay SR2 is energized within the release time of the relay SRI, a circuit is completed as aforesaid from the terminal 55 of the voltage source B through conductor I4, terminal ll, conductor ll, contact 1| of relay SRI, conductor ll, contact 19 of relay SR2, conductor 92 and contacts 14 and 11 of the relay R to the conductor 99, Figs. 5 and 2, and thence through the field coil 2| of the motor M, winding 24 of relay 2!, conductor 2., rheostat 21, conductor 29, terminal 49, and conductor 19 to the terminal 59 of the source of voltage B. This results in the energization of the tield coil 2| and of the relay 2! to thereby initiate reverse rotation of the motor M and this is effective to adjust the speed of the engine E2, for example,

by varying the pitch of propeller P2, in such a manner as to tend to correct the rotative speed of the propeller P2 relative to the propeller P l As explained hereinabove in connection with the operation for correcting low relative velocity of the propeller P2, the just described operation for correcting high relative velocity of this propeller entails continuous operation of the motor M at a comparatively high relative velocity of the propellers, but an intermittent actuation of the motor M while a state of substantial synchronism is being approached, such latter operation of the motor M being repeated often enough to bring the propeller P2 and the engine E2 into substantial synchronism with the master prodo'peller PI and the master engine El, it being recalled that the release times of relays SRI and SR2, Fig. 5, are such that the energizing circuit to a motor field coil as 2| cannot be maintained throughout the first complete turn of the brush 5 41 of the switch SI relative to the brushes of the switch S2. The only substantial difference between the Just described operation of the control unit CU for correcting high relative velocity of the propeller P2, as compared with the operation for correcting low relative velocity of this propeller, is that the relay R remains deenergized at all times in the former operation inasmuch as the master brush 41 will always electrically contact the brush 50 to energize the re- I lay SR2 at a time when the relay R is in a deenergized state and, unless substantial synchronization is effected after the brush 41 breaks contact with the brush 50 and while it remains in electrical contact with the brush 49, the relay SR2 will remain energized to keep its contacts 94 and I open at least until the brush 49 moves out of electrical contact with the brush 41 so that an energizing circuit cannot be completed to the winding 91 of relay R.

It will be appreciated from the foregoing description that when the brushes of the secondary switch S2 are rotating clockwise relative to the brush 41 of the master switch SI, as viewed in Fig. 4, the relay R normally should remain deenergized to thereby condition a circuit to the reverse iield coil 2| of the motor M. On the other hand, if the brushes of the switch S2 are rotating counterclockwise relative to the brush 41 of the switch SI, the relay It should be energized to enable the forward ileld coil 20 of the motor M to aasaeae be energized. It is to be noted in this connection that a condition of substantial synchronization might exist while the brush 41 is in electrical contact with the brush 50 but is out of electrical contact with the brush 49 so that the relay SR2 alone is energized. Then, if the propeller P2 were to commence relative rotation in such a direction that the brushes 48, 49 and W of the switch S2 start to rotate counterclockwise relative to the brush 41 of the switch SI, as viewed in Fig. 4, the brush 59 would move out of electrical contact with the master brush 41 and the brush 49 would be brought into electrical contact with the brush 41 to thereby energize relay SRI, and if this occurred within the release time of the relay SR2, a delay in the operative response of the control unit CU would be effected inasmuch as the relay R is not energized at the time the relays SR| and SR2 are maintaining their contacts 1| and 19 closed concurrently, so that an energizing circuit would be momentarily established from the voltage source B through the aforesaid contacts 1| and 19 to the contacts 19 and 11 of the relay R and thence through the conductor 99 to the reverse field coil 2| of the motor M. It has also been noted hereinabove, however, that the release time of the relay SR2 is such that even under the most extreme conditions that might be foreseen in practice, the relay SR2 will not remain energized throughout the interval between two successive electrical contacts of a brush 50 or 48 with the brush 41 during the first relative turn of these brushes. Moreover, the apparatus should so operate that under such extreme conditions the relay SR2 would open its contact 19 and close its contacts 64 and 65 prior to the time the brush 49 moves into electrical contact with the brush 41.

Thus, the release times of the relays and the brush widths may be so selected that under any circumstances the relay SR2 is fully deenergized at the time brush 41 electrically contacts the brush 49 during the first counterclockwise rotation of the brush 50 of switch S2 relative to the brush 41 of the switch SI, Fig. 4. Hence, despite any operative delay which might be experienced under the conditions just described, relay R will thereafter become energized and if the condition of counterclockwise relative rotation of the brushes of switch S2 persists, the relay R will be energized each time the relay SR2 assumes a state of energization, as described hereinabove, and circuit is thus conditioned to the forward field coil 29 of the motor M.

It will be understood that the control unit CU, Figs. 5 and 1, is associated with the switches SI and S2 for controlling the speed of the engine E2 relative to the master engine El. If there are additional engines to be synchronized with the master engine E| on the aircraft, each such engine is provided with a switch such as S9 or L4 and a contro l unit identical with the unit CU is associated with such switch and the master switch SI for controlling the operation of a motor similar to the motor M associated with the particular engine to be synchronized.

In thecircuit shown in Fig. 4, there is a possibility, particularly where there are a. large number of engines to be synchronized, that a bridging condition may occur when the brushes on the various commutators assume certain relative positions. To avoid this feature, an arrangement shown in Figs. 6 and '7 may be resorted to and by referring to these figures, it will be seen that the master engine El, Fig. 1, has associated therewith a series of master switches SIa, Sla Sla", and so on. Each of these master switches is identical in construction with the switch SI shown in Fig. 4 and the brushes 41a of the various master switches are connected'in parallel by a conductor 82a to a terminal 88a and thence by a conductor 54a to the terminal 85a of the voltage source Ba, Fig. 8, the parts bearing the same reference characters and differing only In the sufilx attached thereto being identical with corresponding parts in Fig. 5. Conductors contained in cables as 5Ia, 5Ia' and ila" interconnect the commutators as 4512 of the master switches as Sla, Sla, and Sla", respectively, with the commutators as 45'a oi the switches 82a, 83a and SM: associated with the engines as E2, Fig. 1, that are to be maintained in timed relation with the master engine EI, the commutator of each switch as S2a being thus interconnected segment for segment with its corresponding master switch commutator as 45a. Each switch as 52a is similar in construction to the corresponding switch as S2 shown in Fig. 4 except that in this instance the brushes as 58a are somewhat wider than are the brushes 58.

With the foregoing arrangement there is no likelihood that undesirable bridging conditions will occur, but in all other respects the apparatus schematically shown in Figs. 6, 7 and 8 operates in the same manner as that shown in Figs. 4 and 5, those elements which perform the same functions in the two circuits being numbered alike except for the suffixes on the reference characters.

In the circuit shown in Figs. 9, 10 and 11 the arrangement of the master switches Slb, Fig. 9, associated with the engine El, Fig. 1, with relation to other commutator switches as S2b, Fig. 10, is generally similar to that shown in Figs. 6 and 7, which is to say that each switch as SR) is interconnected with its corresponding master switch as Slb by conductors in a cable as 5lb The switches as 52b, however, differ in construction from the switches S2 in that each switch is provided with three brushes 85, 88 and 81, which may all be contained within a semi-circle as shown in Fig. 10. Each switch as 521) has associated therewith a control unit as CUb which is somewhat similar in construction to the control unit CU shown in Fig. 5, like elements being numbered with the same reference character in Fig. II, but bearing the suffix b. Thus the brush 88, Fig. 10, is connected by a conductor Bib to one side of the winding 58b of the relay SRIb, the other side of this winding being connected by a conductor 51b to a terminal 56b of the voltage source Bb. The conductor 62b, which is electrically connected to the brush 85 in this instance, is connected to one terminal of a normally closed contact 88 of a quick-to-release relay R2. If, during relative rotation of the brushes 85, 88 and 81 with respect to the brush 4'"), the brush 41b electrically contacts the brush 85 at a time when the relay R2 is deenergized, circuit is extended from the terminal 55b of the voltage source Bb through the brushes 41b and 85 to the conductor 82b and the closed contact 88 and thence through a conductor 89 to a terminal 88 from whence a conductor 9| leads to one side of the winding 82 of another quick-to-release relay RI, the other side of this winding being connected by a conductor 83 to the terminal 58b of the source Bb, and under these conditions the relay RI is energized to open its contact 84 and close its contact 85. Such closure oi. the contact 88 extends circuit from the terminal 88 which is potentialized, so long as brushes 41b and 88 are in electrical contact, through conductor 88, contact 88, conductor 81 to a terminal 88 and thence through a. conductor 88 to a contact I88 of the slow-to-release relay SR b.

Brush 41b is of such width that it can never be in electrical contact with more than one of the brushes 85, 88 or 81 at any instant. Hence, assuming that there is counterclockwise rotation of the brushes 85, 88 and 81 relative to the brush 41b, the just described energization of the relay RI is only momentary, if relay SRIb has not been energized to maintain its contact I88 closed at the time the relay RI is energized, for when brush 41b moves out of electrical contact with the brush 85, circuit from the source Bb to the conductor 82b is broken to remove this source of energization from the winding 82 of the relay RI and the only other source of energization for this winding is through the contact I88 of the relay SRIb which, as has just been explained, is open until the brush 41b electrically contacts the brush 88 during relative counterclockwise rotation of the brushes 85, 88 and 81, as viewed in Figs. 9 and 10 so that relay RI does not remain energized.

When brush 41b is brought into electrical contact with the :brush 88, a circuit is extended from the voltage source Bb through brushes 41b and 88 to the conductor 8Ib which leads to the winding 58b of the slow-to-release relay SRIb, the other side of this winding being connected by a conductor 5') to the source Bb. Relay SRIb thereupon energizes and closes its normally open contacts I88 and Nb. As brush 41b moves out of electrical contact with the brush 86 during continued relative rotation of the brushes, the relay SRIb commences to deenergize but being slow to release it maintains its contacts 'IIb and I88 closed for a predetermined time after deenergization of this relay is initiated. Assuming that the degree of relative rotation of the propellers is such that brush 81 moves into electrical contact with the brush 41b during the aforesaid interval, circuit is extended from the source Bb through the brushes 4'") and 81 to the conductor 63b which leads to the normally closed contact 84 of the relay RI, which relay, as has just been explained, remains deenergized when the brushes 85, 88 and 8'! are rotating counterclockwise relative to the brush 41b and during the time the brush 4'") is not in electrical contact with the brush 85. Hence, circuit is further extended from the contact 84 through a conductor IM to a terminal I82 and thence through a conductor I83 to one end of the winding I84 of the relay R2, the other end of this winding being connected by a; conductor I85 to a terminal 881) which is connected by conductor 18b to the voltage source Bb. Relay R2 thereupon energizes to open its contact 88 and close its contacts I88 and I81. Closure of the contact I88 establishes a circuit from the terminal I82 through a conductor I88, contact I88 and conductor I88 to the terminal 88. It having been assumed that the relay SRIb is maintaining its contact I88 closed at the time relay R2 is thus energized, circuit continues from terminal 88 through conductor 99 and contact I88 to a conductor II8 leading to a terminal III which is included in the conductor 88b so that circuit from the terminal 58b of the source Bb is now completed as described through the winding I84 of relay R2. contact I" of this relay and contact I of relay SRIb back to the terminal 08b of the source Bb, and a holding circuit is thereby established for maintaining the relay R2 energized so long as relay SRib does not release its contact I l0. Inasmuch as the contact III: of relay SRIb is closed while this relay remains in an operative condition, circuit is extended from the terminal 550 of the source Bb through conductor 54!), terminal 50b, conductor 00b, terminal III, conductor II2, contact lib, conductor Bib, terminal Ill, conductor iii and contact I0I of relay R2 to the conductor 23b which leads to a forward field coil as 20 of a motor as M, Fig. 2, circuit being thereafter completed through the conductor 20b back to the terminal 56b of the" source Bb. Thuswhenever the relay R2 is energized prior to the time the relay SRIb has been released, it is held locked up until relay SRIb is released and in so doing it completes an energizing circuit to the field coil as 20 of a motor as M, which motor thereupon functions to correct counterclockwise (as viewed from the rear oi the aircraft) relative rotation oil a propeller as P2, Fig. 1.

I1 brush should electrically contact the brush IIb under these circumstances while the relay SRIb is still maintaining its contacts I00 and lib closed, circuit may be extended from the source Bb through the brushes 41b and 05 and con uctor 62b to one terminal 01 the contact 00 oi t e relay R2, but inasmuch as this relay is being held locked up as aforesaid, circuit cannot be completed to the winding 92 of the relay RI and therefore relay RI remains deenergized, As in the case of the circuit shown in Fig. 5, the slow-to-release relay SRIb is so timed that, under contemplated working conditions. it will not remain energized throughout the entire interval between the first two successive electrical contacts of the brush 06 with the brush l'lb during initiation of relative rotation in a given direction, although it may thereafter remain constantly energized if the relative velocity increases, and hence if there is counterclockwise relative rotation 01' the brushes of the switch 521), Fig. 10, the relays SRIb and R2 may be maintained continually energized at high relative velocities, but at low relative velocities they are repeatedly energized in a manner such as described to repeatedly energize the field coil as 20 of the motor as M, Fig. 2, and as in the case of the control unit CU, Fig. 5, such repeated energizations of a field coil as 2. become increasingly infrequent and smaller in magnitude as a state of substantial synchronization is approached inasmuch as the amount of overlap of the release time or the relay SRIb subsequent to the instant when energized and the holding circuit is established for this relay through its contact 90 and the contact Ill of the relay SRIb in the same manner as the holding circuit was established for the relay R2 during counterclockwise relative rotation oi the brushes of the switch 82b, Fig. 10, with respect to the brush "b, Fig. 9. Thus circuit is extended from source of voltage Bb through the contact lib of relay SRIb, conductor lib, terminal III, conductor Iii and a contact iii, which is closed when the relay RI is energized to conductor 36b which leads to a reverse field coil as 2i of a motor as M, Fig. 2, and the motor M is thereupon actuated to correct the rotative speed of the propeller P2 relative to the propeller PI.

In the arrangement shown in Figs. l2, l3 and 14, a plurality of master switches as Sic, Fig. 12, are employed, which are similar to the master switches Sia shown in Fig. 6, these master switches being connected by individual cables as lie to their corresponding commutator switches as S2c which are associated with the engines to be synchronized. The switches as S2c shown in Fig. 13, however, differ irom any hereinabove described in that only two brushes as I20 and III are included in each of these switches. The brushes as I20 and HI are spaced apart at an angle considerably less than 180 degrees so that there is a much greater travel from one brush to the other brush in one direction of relative rotation than there is in the opposite direction. The control unit CUc, Fig. 14, is identical with the unit CU in Fig. 5 in all respects except that in place of the quick-acting relay R a slow torelease relay SR3 having the same arrangement of contacts is employed and the conductor 620 is connected to conductor Sic which is under control of the brush I20. Assuming that the brushes I20 and I2I are rotating counterclockwise relative to the master brush "c, as viewed in Figs. 12 and 13, the brush I20 may first be brought into electrical contact with the brush 41c and thereupon circuit is completed from terminal 55c of the source 130 through conductor 54c, terminal 530, conductor 52c, brush 41c, conductors in the cable Sic, brush I20 and conductor I22 to a terminal I23 to which is connected a conductor iic leading to one end of the winding 59c of the relay SRIc, the other end of this winding being connected by a conductor 510 to the terminal 56c oi the source 130 and relay SRIc is thereupon energized to close its contact Iic. Circuit is also extended from the terminal I22 through conductor 630 to the contact c of the relay SR2c, which is normally engaged with its cooperating contact 650 and circuit then continues through conductor 660 to relay R2 is energized, is gradually reduced when fi the winding I24 01' the relay SR3 and thence the relative speed of rotation of the brushes'is being diminished.

I! there is clockwise rotation of the brushes 05, 00 and 01 relative to the brush IIb, as viewed in Figs. 9 and 10, the brush 8! may first move into electrical contact with the brush 41b to thereby cause energization of the relay R2 but inasmuch as relay R2 is quick to release it immediately deenergizes when electrical contact between brushes lib and 01 is broken and prior to the instance when brush l6 electrically contacts brush lib to thereby energize relay SRIb. Continued relative rotation of the brushes in this manner brings the brush 05 into electrical contact with brush 01b and it this occurs within the release time of the relay SRIb relay RI is through conductor 68c, terminal 590 and conductor 10c backto the terminal 560 of the source Bc so that relay SR3 is energized simultaneously with the relay SRic. This causes the movable contact ISc of the relay SR3 to move out of engagement with the stationary contact 11c and into engagement with the other stationary contact 100 for conditioning circuit to a forward field coil such as 20 of a motor such as M, Fig. 2.

As the brush I2I continues to rotate counterclockwise relative to the brush lIc, Figs. 12 and 13, brush I2I eventually contacts brush "0 and circuit from the source Be is extended through these brushes to the conductor 52c and thence through the winding 800 of the relay SR2c back to the source 30. Relay SRZc thereupon ener- CJI sizes to separate the contacts 04c and 550 and engage the contact 050 with the contact 12c and if this occurs before the relay SR3 has been released, a holding circuit is established for the winding I 24 of the relay SR3 through the contact 140 of this relay and the contacts 65c and 12c of the relay SR2c. Furthermore, if relay SR2c is energized within the release time of the relay SRIc, a circuit is completed from the source 30 through the closed contact IIc of the relay SRIc, closed contact 100 of the relay SRZc and contacts lie and 10c of relay SR3 to the field coil as 20 of the motor M, Fig. 2, to thereby actuate said motor for correcting the speed-of the engine with which it is associated.

Relays SRIc, SR2c and SR3c are so timed that they will be released, even under extreme operating conditions, in .the interval which elapses while a brush as "c is traversing the larger relative are between the brushes I20 and I2I during the initial relative turn of the brushes in a given direction. Thus if the brush I2I had originally been in electrical contact with brush 410 at the instant the brushes I20 and Ill commenced rotating counterclockwise relative to the brush 410, as viewed in Figs. 12 and 13, relay SR2c would have been initially energized but in the interim following the breaking of electrical contact between brushes and I 21 and the contacting of brush I20 with brush 410, the relay SR2 is released so that whenever there is such counterclockwise relative rotation of the brushes I20 and IN, such prior energization of the relay SR2c is without effect and the sequence of operations of the control unit CUc does not become effective until the relays SRIc and SR3 have been first energized followed by the energization of the re lay SR2c.

When there is clockwise rotation of the brushes I20 and I'2I relative to the brush 41c, Figs. 12 and 13, however, the operative sequence does not commence until the relay SR2c has first been energized upon contact of the brush I2I with the brush 410 for the reason that under these conditions relays SRIc and SR3 will always be deenergized at the time relay SR2c is initially energized. If brush I20 then moves into contact with the brush 410 within the release time of the relay SR2c, relay SR3 will not be energized inasmuch as relay SR2c maintains its contacts 64c and 650 separated so that no energizing circuit can be established through the conductor 53c to the winding I24 of relay SR3, and unless relay SR2c releases while brushes I20 and 410 are still in electrical contact, relay SR3 will not become energized during such clockwise relative rotation of the brushes I20 and I2-I. Relay SRIc energized and closes its contact 'I-Ic when there is contact between the brushes 41c and I20 and if this occurs before the relay SR2c has been released, circuit is extended from the source Bc through the contact 'lIc of relay SRIc, contact 190 of relay SR2c, contacts 150 and 110 of relay SR3 to the conductor 360 which leads to a reverse field coil I2I of a motor as M, and this motor is thereupon actuated to correct the speed of its associated engine as E2, Fig. 1.

It may happen that the brushes I20 and HI will successively traverse the brush "0 at a sufficient rate in one direction of relative rotation to set the control unit CUc for correcting relative rotation in that direction and that operating .conditions may undergo a subsequent change which produces afi opposite relative rotation of the brushes, in which event the setting of the control unit CUc will eilect an operative lag until such setting is sulsequently changed upon the completion of not greater than approximately one full turn of relative rotation in the new direction. Even under extreme conditions, however, such operative lag will be of only slight duration so that it will not interfere with the successful operation of the apparatus as has been explained hereinabove in connection with the foregoing embodiments of the invention.

The arrangement in Figs. 15, 16 and 17 is identical wtih that shown in Figs. 9, 10 and 11 in all respects, except that slow-to-release relays SR2d and SR3d have been substituted for the quickacting relays RI and R2. Also, the conductor 23d is in this instance connected to a terminal of the contact 611 of the relay SR2d and the conductor 30d is connected to a terminal of the contact I0Id of the relay SR3d. When the direction of rotation of the brushes 85d, 86d and 81d is counterclockwise relative .to the master brush "(1 as viewed in Figs. 15 and 16, the brush 41d may originally be in electrical contact with the brush 81d but while brush 41d is passing from the brush 81d to the brush 85d, the relay SR3d if it was original energized is released by the time brushes 41d and 8511 are brought into electrical contact, the release time of relay SR3d being so selected that this will occur during the first relative counterclockwise turn of the brushes in the switch S2d.

Electrical contact of .the brushes 41d and 85d establishes a circuit from the source of voltage Bd through these brushes and conductor 62d to one terminal of the normally closed contact 08d of the relay SR3d and inasmuch as this relay is deenergized circuit continues through the contact 08d and conductor 89d, terminal 90d and conductor Sid to the winding I-2'5 of the relay SR2d and thence through conductor 93d to the voltage source Bd. Relay SR2d thereupon energizes to open its contact 94d and closes contacts 05d and Gil. Continued relative rotation of the brushes as aforesaid brings the brushes 41d and 85d into electrical contact to thereby extend circuit from the source Bd to the conductor IiId for energizing the winding of the relay SRId. If this occurs before relay SRi2d has released, a holding circuit for relay SR2d is established through the contact l00d of relay SRId and contact 55d of relay SR2d to the winding I25 of the latter relay, and hence the relay SR2d remains locked up so long as the relay SRId does not release. As relative rotation continues, brush 81d comes into electrical contact with brush "d and circuit is extended from the voltage source Bd through the brushes to the conductor 53d, which leads to one terminal of the contact 9411 of the relay SR2d. However, if this occurs while the relay SR2d still remains locked up as aforesaid, circuit cannot be completed beyond the contact 34d inasmuch as this contact is held open when the relay SR2d is energized. Thus, in effect, relay SR3d is prevented from energizing so long as relay SRId remains energized during counterclockwise relative rotation of the brushes of the switch 8241, Fig. 16.

Energization of the relays SRI d and SR2d completes an energizing circuit from the source of voltage Bd through the contact Nd and lid of these relays to the conductor 23d for energizing a forward field coil as 20 of a motor as M, Fig. 2, and so long as counterclockwise rotation of brushes 85d, 06d and 31d relative to the brush 41d continues as viewed in Figs. I5 and I5, the

slow-to-release relays SRld and SR2d are alternately energized, to assume a concurrently operative condition either continuously or intermittently, depending upon the degree of relative velocity of the brushes, to either continuously or intermittently energize the motor iield coil as 20 until a state 01' substantial synchronization is reached.

When there is clockwise rotation or the brushes "d, "d and "d relative to the brush "d, the relays SRld, SR2d and SRld will initially be in a deenergized state shortly prior to the establishment oi electrical contact between the brushes 41d and lid, but when such contact is established circuit is extended from the voltage source Bd through these brushes to the conductor 81d and thence through the normally closed contact 84d of relay SRZd, conductor III ld, terminal Ind, and conductor lid, to the winding I26 oi relay SRld and thence through conductor llid, terminal 69d and conductor 'Ild back to the source of voltage Bd so that relay SR3d is energized to maintain its contact 8Id open and its contact lid and "lid closed for a predetermined time after the brush lld moves out of electrical contact with the brush "d. If the degree of relative rotation is great enough so that brush 41d comes into electrical contact with the brush 86d during this interval, circuit is established through these brushes for energizing the relay SRld and upon energization of this relay a holding circuit is established for the winding I26 of relay SR3d through the contact lflGd of this relay and the contact llllld of relay SRld so long as the latter relay remains in a state of energization. Opening of the contact 08d upon energization of the relay SRSd prevents the relay SR2d from energizing when brushes lid and 85d come into electrical contact, provided relay SRld is still energized to maintain the holding circuit for the relay SRSd. Concurrent operation of the relays SRld and SR3d completes circuit from source of voltage Bd through the contacts lid and lid of these relays to the conductor 36d which leads to a reverse field coil as 2| 01' a motor as M, Fig. 2, and this circuit is either maintained continuously or is repeatedly established for every relative turn of the brushes in switch S2d with respect to the master brush "d in the switch Sid, depending upon the value of the relative velocity, until the motor M has brought the engine which it controls back into substantial synchronism with the master engine.

It is desirable when the brushes of the switch S2d are rotating counterclockwise relative to the master brush 41d, that the relay SR3d be prevented from energizing. Of course, there is always the possibility that there may be a slight clockwise relative rotation of the brushes 81d and 86d which is suilicient to cause energization and locking up of the relay SR3d and that the relative rotation of the brushes may then be suddenly reversed'so that the brushes lid and 81d commence to turn counterclockwise relative to the master brush "d and under these circumstances the relay SRIld may remain energized for a brief period during such counterclockwise rotation relative to the master brush "d. The release time of relay SRId is sufliciently limited so that this relay releases and breaks the holding circuit to the winding of the relay SRld on the first counterclockwise relative turn or the brushes of the switch S2d, even under extreme conditions, at least by the time brush 41d has moved into contact with the brush 85d so that the operative delay effected as a result of the energized condition of the relay SR3d is promptly rectified when relay SR3d releases and closes its contact 88d, to thereby enable circuit to be extended from the source of voltage Bd through the brushes 41d and lid to the winding of relay SR2d and thereafter the operation of the control unit CUd is the same as described hereinabove for relative counterclockwise rotation of the brushes 85d, 85d and 81d.

In all the foregoing embodiments of my invention, the control unit such as CU, Fig. 5, has included therein at least one relay such as SR2 which is common to the means for determining the direction of relative rotation of the brushes and the means responsive to the magnitude of such relative rotation. Thus in Fig. 5 not only does the relay SR2 cooperate with the relay R to condition circuit to one or the other of the field coils 20 or 2| of a motor as M, Fig. 2, depending upon the sense of the relative rotations between the brushes of the master switch as SI, Fig. 4, and the switch as S2, but relay SR2 also cooperates with the relay SRI to complete the thus conditioned circuit whenever there is a predetermined rate of such relative rotation. This renders the sensitivity of the relative direction determining means comparable to that of the means responsive to the magnitude oi relative speeds of rotation of the engines as El and E2, Fig. 1. It may be desirable in some instances that the relative direction determining means be considerably more sensitive than is the means responsive to magnitude of relative rotation and for this purpose a circuit such as is exemplified in Fig. 18 might be utilized. This arrangement is generally equivalent to that shown in Figs. 4 and 5 except that each master engine as El, Fig. 1, has associated therewith at least two master switches as Sly and Slz, and two commutator switches as $211 and S22 are provided for each engine as E2. The segments of the switches as Sly and S211 are interconnected by a cable Sly, and those the switches Slz and S22 by a cable I2. The brushes 48y and My of the switch S23 cooperate with the master brush 41y of the switch Sly for operating the slow-to-release relays SRl 1 and SE21: which are similar to and function for the same purpose as the relays SRI and SR2, Fig. 5, in so far as these relays are responsive to the magnitude oi rotation of the brushes I811 and 501/ relative to the brush 41y. The brushes 492 and 502 of the switch S22 cooperate with a master brush "2 to operate the relays SR2z and R2 which are similar to and function the same as the relays SR2 and R, Fig. 5, in so far as these relays are responsive to the sense of rotation of brushes I92 and 502 relative to the brush 412.

The brushes of the switches Slz and S22, however, may be geared to rotate at a much higher speed than the brushes oi the switches Sly and S211 and thus any discrepancy in engine speeds is, in effect, magnified in so far as the switches Slz and S22 are concerned. The release time of the relay SE22 can be so adjusted as to accommodate this increased sensitivity and afford a more rapid response of the relay R2 to discrepancies in engine speeds. Hence the relay Rz will condition an energizing circuit to a selected one of the field coils or 2| of the motor M, Fig. 2, sufficiently far in advance of a time when the relays SRly and SRZy are operated under control of the switch 82y when there is a difference in relative rotative speeds of the r0- tative parts such as propellers PI and P2, Fig. l,

that all likelihood of operative delay is eliminated.

It may be desirable, particularly where there are a large number of engines or the like to be correlated, to reduce the number of conductors required in the cables such as 5|, Fig. 1, that interconnect the master switch as SI with each of the subordinate switches as S2. To this end, it will be feasible in some instances to arrange the conductors as shown in Fig. 19 so that each segment of each switch as SI) or S2 is in electrical contact with the segment 180 degrees removed from it. This reduces the number of conductors required in a cable as 5If to one-half the number required in circuits as shown in Fig. 4. However, this also means that the master brush as 41) of the switch Slf is brought into electrical contact not only with those segments of a switch as S2) with which it would be in electrical contact in an arrangement as shown in Fig. 4, but also with those segments of the switch as $21 which are 180 degrees removed from the first-named segments of this switch. This will necessitate a somewhat diiferent spacing of the brushes of a switch as 82] than would otherwise be required, and each relay as SRIc, Fig. 14, under control of one of these brushes will be energized a plurality of times (in the present instance, twice) in the course of a single turn of the master brush 4' relative to the brushes in the switch S2f. Taking for example the arrangement of the apparatus shown in Figs. 13 and 14, the brushes I20 and I2I would with I the new arrangement be placed closer together, as are the brushes I20 and I2l,f, and the master brush 41 can be likewise made narrower than the brush "a, Fig. 12, so that the relative arc transversed by the master brush I from the instant it moves out of electrical contact with either of the brushes I 20; or I2If to the time when it moves into electrical contact with the other of these brushes (considering only the smaller arc) is appreciably less than 90 degrees. The relays as SRIc, SR2c and SR3, Fig. 14, which are under control of brushes as I20 and I2If, Fig, 19, have their release times adjusted to accommodate this small relative travel of the master brush 411 but will not respond to effect any premature energization of a relay as SR2c if, for example, the brush "I being originally in electrical contact with the brush I201 then commences to rotate counterclockwise relative thereto, as viewed in Fig. 19, to thereby produce the same elfect as though a diametrically opposite brush 41f were to be brought into electrical contact with the brush I2If, the relative length of travel of a brush as 411" from the position in which it is shown in Fig. 19, to the brush I2I being sufliciently great under these circumstances so that the relays as SRIc and SR3 may deenergize on the first relative turn of the brushes before the brush 41)" makes contact with the brush I2If.

As has been explained hereinabove in connection with the various embodiments of my invention, the release times of slow-to-release relays such as SR2, Fig. 5, or SRIb, Fig. 11, are so selected that each such relay is unable to remain in an operative condition during at least the first two successive electrical contacts established between the master brush and the brush to which such slow-to-release relay is electrically connected, upon initiation of contemplated relative rotations of the brushes. The time factor thus incorporated in the operation of these relays insures against misoperation of the control apparatus when there is a sudden reversal in the sense of relative rotation of the propellers under conditions which will be encountered in the normal use of aircraft. For example, in the embodiment illustrated in Figs. 9 to 11, the brushes 86 and 81 might be successively brought into electrical contact with the master brush 41 while rotating counterclockwise relative thereto, to thereby energize the relays SRIb and R2 for correcting a low rotative speed of the propeller as P2, Fig. 1, relative to the master propeller as PI, and there might then occur some change in operating conditions which causes the propeller P2 to rotate more rapidly than the propeller PI, so that the sense of relative rotation of the brushes is reversed. Hence, assuming relays SRIb and R2 to have been energized as aforesaid, the brushes 85, 86 and 81 thereupon commence to rotate clockwise relative to the master brush 41b, as viewed in Figs. 9 and 10, and upon the establishment of electrical contact between brushes 41b and 86, relay SRIb is reenergized. If this occurs prior to the release of the relay SRIb following its previous energization, the relay R2 remains locked up through its holding circuit controlled by the contact I00 of relay SRIb, and relay RI is prevented from energizing, until such time as relay SRIb releases, notwithstanding that relay RI and not relay R2 should be energized under these circumstances.

It is contemplated that even under the most extreme conditions which will be encountered in practice, relay SRIb will be released prior to the instant when brush 86 electrically contacts brush 411) a second time while rotating clockwise relative thereto, so that relay R2 will be released due to opening of the contact I00 of relay SRIb during the first relative clockwise turn of the brush 86. This enables relay RI to subsequently become energized and to lock up, when relay SRIb is again energized and closes its contact I00, and relay R2 is thereafter prevented from energizing so long as relay SRIb is in an operative condition. However, it may be desirable to provide an absolute safeguard against the possibility, remote as it may be, that the relays SRIb and R2 will be caused to assume an operative condition for correcting low relative velocity of the propeller P2, and that the relative velocity of the propeller P2 may then increase at such a rate that brush 86 repeatedly contacts brush 41b, with suflicient rapidity to maintain the relay SRIb in a continuously operative condition, thereby holding the relay R2 in its locked-up condition and preventing the relay RI from energizing. Because of this, I have provided a form of my invention, as illustrated in Fig. 20, which positively insures against the occurrence of an improper condition as just described.

The control CUg shown in Fig. 20, is similar in many respects to the control unit CUb, Fig. 11, and is adapted to be operated under the control of a commutator switch as 82b. Fig. 10. In the present instance, however. the conductor 51g leading from the winding 50g of the relay SRlg is not connected directly to the source of voltage Bg but leads to one terminal of the contact I30 of the relay Rlg. The contact I30 is normally closed when relay Rlg' is deenergized and serves to connect the conductor 51g with a conductor I3I that leads to one terminal of a like contact I32 of the relay R2g, which contact normally connects the conductor l3l to a conductor I" leading to a terminal Ill, and thence circuit is completed through the conductor 10g to the terminal 58g of the voltage source By. Whenever a relay as Rig or R20 is energized, however, the energizing circuit for the winding 59;! of the relay SRlg is broken due tothe' opening of either of the contacts I30 or I32. Hence, if as in the above described example of operation, relay Rig is energized while the relay SRig is in an operation condition, such as would occur when there is counterclockwise relative rotation of the brushes of the switch as 82b, Fig. 10, the contact I32 remains open so long as relay R2g is held locked up through its holding circuit, which is to say, so long as relay SRla remains in an operative condition. Then, if the sense of relative rotation of the brushes should be suddenly reversed, and as brush llb, Fig. 9, moves into electrical contact with the brush as 86, the relay SRlg will nevertheless continue to deenergize and will subsequently be released to thereby break the holding circuit to the relay RZg regardless of how long or how often the brush as 86 is in electrical contact with the master brush as 41b when there is relative clockwise rotation of the brushes as 86. When relay R29 has been released to close its contact I32, the energizing circuit for the relay SRig is again conditioned for operation and thereafter the relay SRlg and then the relay Rlg may become energized to bring about the necessary control operation for correcting the high relative speed of the propeller as P2, Fig. 1.

To afford still further reliability of operation, the conductors 93g and I050 leading from the windings 92g and llllg, respectively, of the relays Rio and R29 have been connected to the conductor 99g which leads to one terminal of the normally open contact "log of the relay SRia, the other terminal of this contact being connected by a conductor I35 to the terminal 56g of the volt age source By. The conductors 91g and I090 leading from corresponding terminals of the normally open holding-circuit contacts g and lllSg of the relays Rlg and RZg, respectively, are connected to a conductor I36 leading to the terminal 55g of the voltage source By. This arrangement insures that neither the relay Rig nor the relay R2g can be energized so long as the relay SR: is in an inoperative condition, and hence, all control operations must begin with the operation of relay SRlg.

For example, if upon initiation of counterclockwise relative rotation of the brushes of the switch as 8%, Fig. 10, the master brush as 41b should first be brought into electrical contact with the brush as 85, the relay Rlg will not be energized inasmuch as the energizing circuit through its winding 92;; cannot be completed until the contact I000 of relay SRI g is closed, which is to say,

until relay SRIg is energized to assume an operative condition. If however, the relay SRlg becomes energized upon subsequent contacting of the brush as 86 with the master brush as "b, and while SRlg is maintaining its contact IMg closed the brush as 81 should electrically contact the master brush as 41b, circuit from the voltage source By is extended through the brushes as 41b and 81, conductor 639, closed contact 94g of the deenergized relay Rig, and conductors llllg and lllig to the winding Hllg of relay Rig, and thence through conductors i059 and 999, closed contact 10017 of the relay SRlg, and conductor I35 back to the voltage source By. Relay RZg thereupon energizes and closes its contact I069 to thereby establish a holding circuit from the terminal a of the source By through the conductors I38 and I", contact I 08c, and conductor I030 to the winding Illa, which holding circuit continues through the contact lllllg of relay SRig as Just described, so long as relay SRI g maintains its contact closed. Energization of the relay R29 opens the contact Sly to thereby prevent the relay Rig from energizing, as in the embodiment shown in Fig. 11, and also opens the contact I32 which is effective as aforesaid to prevent the relay SRlg from being subsequently energized until it has first assumed an intervening inoperative condition.

It will be appreciated from the foregoing that relay SRlg cannot remain operative for more than a predetermined time following each energization thereof, unless it should so happen that a condition of substantial synchronism is attained while the brushes as 41b and 86 are in electrical contact. Therefore, irrespective of how great the magnitude of the relative velocity of the propellers as PI and P2 may be, the relay SRig cannot remain in a continuously operative condition and hence even at high relative velocities, the control unit CUg may furnish a series of control impulses to the motor as M, Fig. 2, although the amount of overlap in the release time of the relay SRIg following the subsequent energization and locking up of a relay as Rl'n or R20 will vary in accordance with the magnitude of such relative rotation as in the case of the control unit CUb, Fig. 11.

It may be desirable to eliminate the pulsing control afforded by the control unit 0119, Fig. 20, at high relative speeds of the propellers without affecting the pulsing control at lower speed differentials, which, as has been explained hereinabove, aids to prevent hunting when the propellers are being brought into substantial synchronism. To this end, I have shown in Fig. 21 a modification of the circuit arrangement disclosed in Fig. 20. This modification consists of incorporating the slow-to-release relay SR4 in the circuit for maintaining the connection from the terminal 56h of the voltage source Bh to the winding 59h of the slow-to-release relay SRlh continuously closed at high relative speeds. It will be understood that the control unit CUh, which is fragmentarily shown in Fig. 21, is identical in all respects with the control unit CUg, except for the addition of the relay SR4, like parts bearing similar reference numbers differing only in the sufllx added thereto.

The control unit 0111:. is under the control of a commutator switch $271. that is similar in construction to the switch 52b, Fig. 10, except that a fourth brush I" is added to the motor assembly, this brush being preferably located diametrically opposite the brush 8511. which controls the relay SRIh. When the brush "0 is brought into electrical contact with the master brush as "b, Fig. 9, circuit is completed from the voltage source Bh through said master brush and the brush illl, Fig. 21, to the conductor I that leads to one end of the winding ll! of the slowto-release relay SR4, thence through conductor 3, terminal I, and conductor 5 to a terminal I which is included in the conductor 5h leading back to the voltage source Bli. Relay SR4 thereupon energizes and closes its normally open contact I" to thereby establish an electric connection from the terminal I and conductor I 48 to the conductor 9 which leads to the terminal l5! that is included in the conductor 51h connected to the end of the winding 59h of relay 

